Allowing case insensitive regexp groups after PR #144

[jprobinson]

Changes in #144 broke regexp matching for case insensitive groups.

This adds a test scenario and a small hunk of code reverted to what it was prior to the PR.

[jprobinson]

Benchmark changes:

benchmark                             old ns/op     new ns/op     delta
BenchmarkMux-4                        2712          2569          -5.27%
BenchmarkMuxAlternativeInRegexp-4     3835          3860          +0.65%
BenchmarkManyPathVariables-4          4410          4476          +1.50%

[elithrar]

Thanks @jprobinson—and thanks for adding the benchmarks (which I was just about to ask for). Happy to lose a few % if it means we maintain API compat, especially since request muxes are almost never bottlenecks in real applications.