Fixed Scheme validation to reject schemes with non-alphanumeric characters

[JohnJamesUtley]

Background

RFC 3986 defines a scheme as follows

scheme        = ALPHA *( ALPHA / DIGIT / "+" / "-" / "." )

The Bug

Furl will accept non-alphanumeric characters in its schemes

import furl
f = furl.furl('sch^eme://user@host.com')
print(f.scheme)

In this example, Furl will currently print the scheme as 'sch^eme'. This is in violation of RFC 3896 because of the '^' character.

The Pull Request

• Has Furl's scheme setter confirm the schemes validity
• Fixes is_scheme_valid to actually check if the scheme is valid
  • Used to always return true even if the regex did not match
• Adds tests to confirm that Furl will no longer accept schemes with invalid characters

[kenballus]

Suggested change

	if scheme is not None and scheme != '':
	if is_valid_scheme(scheme) == False:
	raise ValueError("Invalid Scheme: " + scheme)
	if scheme and not is_valid_scheme(scheme):
	raise ValueError("Invalid scheme")