Fixes #426
Extends #442

This PR is majorly based on @sd2k's work in #442 and adds support of Content-Type: application/graphql POST requests to all integration crates.

Also, some refactorings are made across codebase for better readability, and removing redudant boxing here and there.

juniper

• Refactor HTTPIntegration to support application/graphql requests:
  • rename HTTPIntegration as HttpIntegration;
  • rename post as post_json method;
  • add post_graphql method.
• Add test_graphql_post and test_invalid_graphql_post.
• Update CHANGELOG/docs/examples.

juniper_actix

• Update tests to support new HttpIntegration.
• Fix application/graphql POST request support:
  • At the moment it deserializes request's body as JSON, which is not correct.
• Update CHANGELOG/docs/examples. (initial release, so not CHANGELOG yet)

juniper_hyper

• Update tests to support new HttpIntegration.
• Run tests for both sync and async versions.
• Support application/graphql POST request:
  • Return 400 Bad Request if Content-Type is absent or invalid.
• Make async handler a default one (rename graphql_async -> graphql and graphql -> graphql_sync).
• Update CHANGELOG/docs/examples.

juniper_iron

• Update tests to support new HttpIntegration.
• Support application/graphql POST request:
  • Return 400 Bad Request if Content-Type is absent or invalid.
• Update CHANGELOG/docs/examples.

juniper_rocket

• Update tests to support new HttpIntegration.
• Support application/graphql POST request:
  • Forward request if Content-Type is absent or invalid.
• Update CHANGELOG/docs/examples.

juniper_rocket_async

• Update tests to support new HttpIntegration.
• Support application/graphql POST request:
  • Forward request if Content-Type is absent or invalid.
• Update CHANGELOG/docs/examples.

juniper_warp

• Revive HttpIntegration tests (were commented).
• Run tests for both sync and async versions.
• Support application/graphql POST request:
  • Omit executing request if Content-Type is absent or invalid.
• Update CHANGELOG/docs/examples.