Properly encode/decode parameter values

[gjdev]

Allow for path parameters with less trivial values, containing special characters, path-part separators, etc.

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[gjdev]

I signed it!

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We found a Contributor License Agreement for you (the sender of this pull request) and all commit authors, but as best as we can tell these commits were authored by someone else. If that's the case, please add them to this pull request and have them confirm that they're okay with these commits being contributed to Google. If we're mistaken and you did author these commits, just reply here to confirm.

[gjdev]

I did author those commits, it's just another email address.

[nevir]

Awesome, thanks!