Convert HTTPS response to JSON and return

Changes:
    - This is needed for test caller to be able to access to JSON
      data from login session.

Example of the response:
{
   '@odata.id': '/redfish/v1/SessionService/Sessions/ZZNZ3SYN0i',
   '@odata.type': '#Session.v1_0_2.Session',
   'Description': 'Manager User Session',
   'Id': 'ZZNZ3SYN0i',
   'Name': 'User Session',
   'Oem': {
       'OpenBMC': {
           '@odata.type': '#OemSession.v1_0_0.Session',
           'ClientID': '123456',
           'ClientOriginIP': '::ffff:xx.xx.xx.xx'
        }
   },
   'UserName': 'root'
}

Tested:
    By adding a piece of test code as
    ${resp}=  Redfish Login  kwargs= "Oem":{"OpenBMC" : {"ClientID":"123456"}}
    Log To Console   \n Session ID: ${resp["Id"]}
    ${resp}=  Redfish Login
    Log To Console   \n Session ID: ${resp["Id"]}

Output:
   Session ID: RNUnVGoyIF
   Session ID: ipDXBgxDjg

Change-Id: I14ce07287ba3c30761e1c8f1a28c34a74d87e9c7
Signed-off-by: George Keishing <gkeishin@in.ibm.com>
diff --git a/lib/rest_client.robot b/lib/rest_client.robot
index 423ab0d..2b70d54 100644
--- a/lib/rest_client.robot
+++ b/lib/rest_client.robot
@@ -396,6 +396,8 @@
     ...  data=${data}  headers=${headers}
     Should Be Equal As Strings  ${resp.status_code}  ${HTTP_CREATED}
 
+    ${content}=  To JSON  ${resp.content}
+
     Set Global Variable  ${XAUTH_TOKEN}  ${resp.headers["X-Auth-Token"]}
 
-    [Return]  ${resp}
+    [Return]  ${content}