clarify extraneous characters after names

[cvazac]

It is currently unspecified as to whether extraneous characters following metric-name should be ignored.

We ignore extraneous characters found after a server-timing-param-value, so I'm inclined to do the same for metric-names.

Example:

Server-Timing: metric=;dur=123.4

Note: = is not a legal token character.

Options:

1. parsing should "keep going", yielding:
   name=metric
   duration=123.4

2. parsing should stop, yielding:
   name=metric
   duration=0

cc @yoavweiss

[yoavweiss]

I think it makes sense for us to "keep going", drop parsing when a non-token-char is hit and skip till the next delimiter