Implement last-match-wins behaviour in mux

[johanbrandhorst]

In accordance with the http.proto spec, we should
use last-match-wins to determine which handler
is chosen if two are matched.

See #224
for more discussion.

[codecov-commenter]

Codecov Report

Merging #1384 into v2 will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##               v2    #1384      +/-   ##
==========================================
- Coverage   58.95%   58.81%   -0.14%     
==========================================
  Files          32       32              
  Lines        3927     3904      -23     
==========================================
- Hits         2315     2296      -19     
+ Misses       1358     1354       -4     
  Partials      254      254              

Impacted Files	Coverage Δ
internal/descriptor/registry.go	59.61% <ø> (+0.90%)	⬆️
...c-gen-grpc-gateway/internal/gengateway/template.go	79.79% <ø> (-0.79%)	⬇️
runtime/mux.go	58.87% <100.00%> (-2.19%)	⬇️
runtime/pattern.go	88.43% <100.00%> (-0.67%)	⬇️

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