Malformed URL can be written to STIX.

[delliott90]

Describe the bug
According to the STIX spec, URLs must confirm to RFC3986. STIX-shifter can currently write data to the url.value property that doesn't conform to this standard. For example if the URL is missing http:// or https://

Expected behavior
Validation should run on the data mapped to the url.value property before being written to STIX. If the data fails validation it should be omitted from the results.

[pcoccoli]

If the scheme is missing, would it make sense to just add http:// in front of the value?

[delliott90]

The problem is we wouldn't know if we should add http:// or https://