Use the original instance_url for soap auth

The explicitly set server/instance URL was being overriden by what the server returned.
There are however probably good reasons for the original URL to be set by the developer.
Ignoring the developer's decision without giving an option to override,
 forces the developer to copy-paste and rewrite code that uses the method.

This commit makes that unnecessary.

Fixes #21 : SOAP auth overwrites instance URL

salesforce_api/login.py

@@ -97,9 +97,7 @@ def soap(instance_url: str, username: str, password: str = None, security_token:
 
     if response.has('soapenv:Envelope/soapenv:Body/loginResponse/result/sessionId'):
         session_id = response.get_value('soapenv:Envelope/soapenv:Body/loginResponse/result/sessionId')
-        server_url = response.get_value('soapenv:Envelope/soapenv:Body/loginResponse/result/serverUrl')
-        instance = re.match(r'(https://(.*).salesforce\.com/)', server_url).group(1)
-        return plain_access_token(access_token=session_id, instance_url=instance, session=session, api_version=api_version)
+        return plain_access_token(access_token=session_id, instance_url=instance_url, session=session, api_version=api_version)
 
     if not response.has('soapenv:Envelope/soapenv:Body/soapenv:Fault/faultcode'):
         raise exceptions.AuthenticationError