Fix a regression in connection cleanup.

We don't test that connections actually get closed, and this was
broken by a refactoring that dropped close() on Connection.
Merge pull request #897 from square/jwilson_0605_regression_connection_close

Fix a regression in connection cleanup.
Make the content type optional.

https://github.com/square/okhttp/issues/899
Add a Headers recipe.
Merge pull request #901 from square/jwilson_0608_headers_recipe

Add a Headers recipe.
Merge pull request #900 from square/jwilson_0608_nullable_content_type

Make the content type optional.
Import mimecraft as-is.
Convert mimecraft types to use OkHttp types: RequestBody, MediaType.

This drops the Part type, replacing it with Headers+Body in MultipartBuilder.