Fix inconsistent URL encoding

[kzantow]

Fixes three issues with the encoding/decoding of strings:

1. Namespace segments are escaped as paths instead of queries to match
   how they are unescaped.
2. Subpath segments are escaped to match how they are unescaped.
3. Names are unescaped to match how they are escaped.

A test is added to ensure this is enforced for all components requiring
encoding. Five tests cases failed previous to the change:

1. unencoded_namespace_segment_is_encoded:
   expected pkg:type/name/spac e/name@version?key=value#sub/path
   to parse as pkg:type/name/spac%20e/name@version?key=value#sub/path
   but got pkg:type/name/spac+e/name@version?key=value#sub/path
2. pre-encoded_namespace_segment_is_unchanged:
   expected pkg:type/name/spac%20e/name@version?key=value#sub/path
   to parse as pkg:type/name/spac%20e/name@version?key=value#sub/path
   but got pkg:type/name/spac+e/name@version?key=value#sub/path
3. pre-encoded_name_is_unchanged:
   expected pkg:type/name/space/nam%20e@version?key=value#sub/path
   to parse as pkg:type/name/space/nam%20e@version?key=value#sub/path
   but got pkg:type/name/space/nam%2520e@version?key=value#sub/path
4. unencoded_subpath_segment_is_encoded:
   expected pkg:type/name/space/name@version?key=value#sub/pat h
   to parse as pkg:type/name/space/name@version?key=value#sub/pat%20h
   but got pkg:type/name/space/name@version?key=value#sub/pat h
5. pre-encoded_subpath_segment_is_unchanged:
   expected pkg:type/name/space/name@version?key=value#sub/pat%20h
   to parse as pkg:type/name/space/name@version?key=value#sub/pat%20h
   but got pkg:type/name/space/name@version?key=value#sub/pat h