Decode path-encoded URL components

[v3n]

This change causes pct-encoded characters passed via path parameters to
be correctly decoded as described in google.api.http (see: path template
syntax) and as implemented in grpc-http-json-transcoding.

A new configuration option is introduced, WithDecodeMode(), which
understands several modes. Backwards compatibility is maintained, with
the hope of UnescapingModeAllExceptReserved becoming the default mode in
V3.

References to other Issues or PRs

Fixes #660

Similiar to #2259, with a few significant differences: preserves backwards compatibility and strictly implements the spec describe in google.api.http and implemented in grpc-http-transcoding.

Have you read the Contributing Guidelines?

Yes.

Brief description of what is fixed or changed

• Backwards compatibility is maintained via UnescapingModeLegacy, which short-circuits the path encoding flow.
• Strictly implements the spec as described in the google.api.http, if the template is multi-segment, %2F should not decoded into /.
• Follows the reference implementation at grpc-http-json-transcoding
  • Provides multiple user-configurable decoding modes (UnescapingModeAllExceptReserved, UnescapingModeAllExceptSlash, UnescapingModeDefault, and UnescapingModeLegacy)
  • Decoding occurs only for variable binding, routing does not use decoding.

Other comments

n/a

[adambabik]

Thank you for the contribution, it looks really solid!

The only thing I can comment on is to add some test cases. For the changes in runtime/mux.go, you can extend TestMuxServeHTTP. For the runtime/pattern.go there is TestMatch.

[v3n]

Added tests, I think ready for merge unless there are more follow-ups?

[codecov-commenter]

Codecov Report

Merging #2332 (5f013c0) into master (93a4e33) will increase coverage by 0.19%.
The diff coverage is 67.96%.

@@            Coverage Diff             @@
##           master    #2332      +/-   ##
==========================================
+ Coverage   59.56%   59.75%   +0.19%     
==========================================
  Files          36       36              
  Lines        4543     4642      +99     
==========================================
+ Hits         2706     2774      +68     
- Misses       1544     1570      +26     
- Partials      293      298       +5     

Impacted Files	Coverage Δ
runtime/mux.go	62.14% <45.45%> (-2.03%)	⬇️
runtime/pattern.go	82.66% <74.07%> (-5.01%)	⬇️

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[johanbrandhorst]

This looks great, a few style comments is all I've got. I'm wondering if you could add a short blurb to our documentation to explain the nuances of these new settings as well? Maybe a section in https://github.com/grpc-ecosystem/grpc-gateway/blob/master/docs/docs/mapping/customizing_your_gateway.md?

[johanbrandhorst]

Fantastic, this looks good to me!

[johanbrandhorst]

Looks like one of the files still needs to be gofmted

[adambabik]

👏

[v3n]

Whoops! Should be fixed now.

[johanbrandhorst]

Thank you for your contribution!