RFR: 625: Proxy build plugin should work on URLs without scheme

[Erik Helin]

Hi all,

please review this patch that makes the build work with http proxy environment variables that do not have a scheme
(i.e. does not have a `http://` prefix).

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - 625

Changes: https://git.openjdk.java.net/skara/pull/809/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=809&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-625
  Stats: 28 lines in 1 file changed: 17 ins; 1 del; 10 mod
  Patch: https://git.openjdk.java.net/skara/pull/809.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/809/head:pull/809

PR: https://git.openjdk.java.net/skara/pull/809