Include a user-agent by default.
Merge pull request #1063 from square/jwilson_0927_user_agent

Include a user-agent by default.
Unused imports.
Merge pull request #1064 from square/adrian.unused-imports

Unused imports.
OkHttpClient.cancel(tag) doesn't cancel in-flight synchronous requests.
Merge pull request #1065 from square/adrian.cancel-by-tag-synchronous

OkHttpClient.cancel(tag) doesn't cancel in-flight synchronous requests.