In Go's header parsing, the same header multiple times results in multiple entries in the `r.Header[...]` slice, but Go does no further parsing beyond that (and in https://golang.org/cl/4528086 it was determined that until/unless the stdlib itself needs it, Go will not do so).
The consequence here for parsing of `Accept:` headers is that we support the way Go outputs headers, but not all language HTTP libraries have a facility to output multiple headers instead of a single list header.
This change ensures that the following (valid) header blocks all parse to the same result for the purposes of what is being tested here:
```
Accept: a/b
Accept: b/c
Accept: d/e
```
```
Accept: a/b; q=0.5, b/c
Accept: d/e
```
```
Accept: a/b; q=0.1, b/c; q=0.2, d/e; q=0.8
```
Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>
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