Prepare for v1.15.0

[jhump]

Below is a draft of the release notes that will accompany this release.

---

What's Changed

Since the last release, we have been significantly increasing coverage of our conformance suite. This has identified several inconsistencies with other Connect, gRPC, and gRPC-Web implementations. This release includes several fixes that will help align this implementation with others, behavior that can be verified by the conformance tests going forward. The fixes are backward-compatible with gRPC and Connect clients and servers. They fix some aspects of the wire representation and edge-case handling of spurious requests and responses, and they may affect users directly inspecting HTTP requests and responses.

Governance

• Add @emcfarlane as a maintainer in Add Ed McFarlane as a maintainer #680 🎉

Enhancements

• Add transport-level retry support for RPCs with unary and server-stream RPCs. This also improves efficiency by eliminating a goroutine and a message copy. By @emcfarlane in Implement unary HTTP calls with retry support #649
• Prefer "application/grpc" over "application/grpc+proto" content-type when issuing gRPC calls. This accommodates Google Cloud Platform servers, which don't fully implement the gRPC specification. By @lrewega in Prefer an implicit codec when using the gRPC default #655
• Small efficiency gains in on all outbound HTTP requests and unary Connect response validation by @mattrobenolt in Replace a Header.Get call with getHeaderCanonical #682, Remove redundant work when validating unary Connect response #683
• Improve efficiency handling gRPC-Web and Connect end-of-stream messages by @jhump in Release buffer for end-stream messages back to the pool #678

Bugfixes

• Fix HTTP status to Code mapping by @emcfarlane in Fix HTTP Status to Code mapping #673
• Client should verify response content-type by @jhump in Client should verify response content-type #679
• Omit erroneous @type attribute from Connect error detail debug string by @jhump in Drop @type from Connect error detail debug string #688
• Use context to correctly classify some errors with Canceled or DeadlineExceeded code by @emcfarlane in Wrap errors with context cancellation codes #659
• Fix trailers-only response handling and some gRPC error coding by @jhump in Only use trailers-only response in grpc-web when there are no custom headers #677, Reconcile trailers-only and misc error behavior with grpc-go #690

Full Changelog: v1.14.0...v1.15.0