Support unvalued options for git/svn daemons.
Support unvalued options for git/svn daemons.

Several options of git/svn daemons are not expecting any value,
and no "=" should be appended to the mnemonic when used.

Note that the way to identify such a parameter is tricky and
debatable. The choice made here is to consider empty strings as a
marker for unvalued parameters. To give an actual empty string to
a parameter, a string with a space can still be used.

For example the following configuration:

{
    ...
    "repositoryCache": {
        "git": {
            "foo": " "
            "verbose": "",
            "syslog": "",
            "max-connections": 42
        }
    }
}

Would make private-bower invoke the git daemon as follows:
  git-daemon --foo=  --verbose --syslog --max-connections=42

(Notice the two spaces after "--foo=", effectively giving an empty
value to the "foo" parameter)

Fixes https://github.com/Hacklone/private-bower/issues/200.

Hacklone/private-bower,criteo-forks/private-bower,Hacklone/private-bower,criteo-forks/private-bower