Don’t unnecessarily escape backslashes in Win32 paths

Under ::CommandLineToArgvW() rules, the backslash character only gets special treatment if it’s immediately followed by a double quote. So, when checking to see if a string needs Win32 escaping, it’s sufficient to check for the presence of a double quote character. This allows paths like "foo\bar" to be recognised as “sensible” paths, which don’t require the full escaping.
Merge pull request #702 from nickhutchinson/feature/win32-escaping-fix

Don’t unnecessarily escape backslashes in Win32 paths