Fix incorrect padding in b64enc and use fetch API

[aryeeteygerald_rogers.com]

Summary

Use fetch API for parsing HTTP_PROXY environment variable
which ensures authentication information is correctly parsed.

Also fix padding bug in b64enc (Base 64 encoding) caused by
off-by-one error.

PR: 129431,153211,234962

Test Plan

b64enc fix was tested against https://en.wikipedia.org/wiki/Base64#Output_padding examples.