Added test for invalid arguments. And merged if checks in set client

auth keyword.

atest/auth.robot

@@ -40,3 +40,14 @@ GET Digest Auth
     Run Keyword And Expect Error  KeyError: 'content-type'  
     ...        GET        ${url}
     Integer    response status    401
+
+Set Auth Invalid Args
+    ${error}   Set Variable       TypeError: Argument "auth_type" must be \${NONE}, basic, digest or proxy.
+    Run Keyword And Expect Error  ${error}
+    ...        Set Client Authentication  none  ${user}  ${password}
+
+    Run Keyword And Expect Error  ${error}
+    ...        Set Client Authentication  doge  ${user}  ${password}
+
+    Run Keyword And Expect Error  ${error}
+    ...        Set Client Authentication  ${1}  ${user}  ${password}

src/REST/keywords.py

@@ -98,19 +98,21 @@ def set_client_authentication(self, auth_type, user=None, password=None):
         | `Set Client Authentication` | ${NONE} | | |
         """
         error_auth = "Argument \"auth_type\" must be ${NONE}, basic, digest or proxy."
-        if auth_type != None and not isinstance(auth_type, str):
-            raise TypeError(error_auth)
-
         if auth_type != None:
+            if not isinstance(auth_type, str):
+                raise TypeError(error_auth)
+
             auth_type = auth_type.lower()
+
+            if not auth_type in ["basic", "digest","proxy"] :
+                raise TypeError(error_auth)
+
             if auth_type == "basic" :
                 auth_type = HTTPBasicAuth
             elif auth_type == "digest" :
                 auth_type = HTTPDigestAuth
             elif auth_type == "proxy" :
                 auth_type = HTTPProxyAuth
-            else :
-                raise Exception(error_auth) 
 
         return self._setauth(auth_type, user, password)