Bump okhttp from 3.6.0 to 3.12.0 in /examples

[dependabot-preview]

Bumps okhttp from 3.6.0 to 3.12.0.

Changelog

Sourced from okhttp's changelog.

Version 3.12.0

2018-11-16

• OkHttp now supports TLS 1.3. This requires either Conscrypt or Java 11+.

• Proxy authenticators are now asked for preemptive authentication. OkHttp will now request
  authentication credentials before creating TLS tunnels through HTTP proxies (HTTP CONNECT).
  Authenticators should identify preemptive authentications by the presence of a challenge whose
  scheme is "OkHttp-Preemptive".

• OkHttp now offers full-operation timeouts. This sets a limit on how long the entire call may
  take and covers resolving DNS, connecting, writing the request body, server processing, and
  reading the full response body. If a call requires redirects or retries all must complete within
  one timeout period.

  Use OkHttpClient.Builder.callTimeout() to specify the default duration and Call.timeout() to
  specify the timeout of an individual call.

• New: Return values and fields are now non-null unless otherwise annotated.

• New: LoggingEventListener makes it easy to get basic visibility into a call's performance.
  This class is in the logging-interceptor artifact.

• New: Headers.Builder.addUnsafeNonAscii() allows non-ASCII values to be added without an
  immediate exception.

• New: Headers can be redacted in HttpLoggingInterceptor.

• New: Headers.Builder now accepts dates.

• New: OkHttp now accepts java.time.Duration for timeouts on Java 8+ and Android 26+.

• New: Challenge includes all authentication parameters.

• New: Upgrade to BouncyCastle 1.60, Conscrypt 1.4.0, and Okio 1.15.0. We don't yet require
  Kotlin-friendly Okio 2.x but OkHttp works fine with that series.

  implementation("org.bouncycastle:bcprov-jdk15on:1.60")
  implementation("org.conscrypt:conscrypt-openjdk-uber:1.4.0")
  implementation("com.squareup.okio:okio:1.15.0")

• Fix: Handle dispatcher executor shutdowns gracefully. When there aren't any threads to carry a
  call its callback now gets a RejectedExecutionException.

• Fix: Don't permanently cache responses with Cache-Control: immutable. We misunderstood the
  original immutable proposal!

• Fix: Change Authenticator's Route parameter to be nullable. This was marked as non-null but
  could be called with null in some cases.

• Fix: Don't create malformed URLs when MockWebServer is reached via an IPv6 address.

• Fix: Don't crash if the system default authenticator is null.

• Fix: Don't crash generating elliptic curve certificates on Android.

• Fix: Don't crash doing platform detection on RoboVM.

• Fix: Don't leak socket connections when web socket upgrades fail.

... (truncated)

Commits

• 7f63a35 [maven-release-plugin] prepare release parent-3.12.0
• 9e195fa Update changelog for OkHttp 3.12.
• 2198975 Merge pull request #4388 from square/jwilson.1114.use_ec_not_ecdsa
• d062472 Specify "EC" to generate eliptic curve keys, not ECDSA
• 1f7e796 Cleanup HttpLoggingInterceptor (#4346)
• ef34a41 Add a LoggingEventListener and use it in okcurl (#4353)
• 8a01554 Merge pull request #4376 from square/jwilson.1107.preemptive_auth
• 5a316c0 Preemptive auth for proxy CONNECT
• 568a91c Merge pull request #4381 from square/jwilson.1112.weaken_immutable
• 764b31b Relax handling of Cache-Control: immutable
• Additional commits viewable in compare view

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