Be more consistent and slightly more lenient when handling GET params

Fixes an inconsistency when passed length of buffer without `\0` terminating
character (ie. "this_key_is_present=&but_this_one_is_not=").

Also handles the "key" without "=" case by ignoring the key.

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The spec is pretty bad about defining how query params should be handled... I need this support for a project I'm working on but I'd love to hear some input on these changes.

Things I didn't figure out fully: (1) whether this library always expects `evhtp_parse_query` to be passed a buffer size *including* terminating `\0` and (2) whether a key without value *and without "="* should be ignored or treated as a key with empty value (can't find a definitive answer/note on the specification).