refactor(constants): Changed constants to static readonly per codacy

Author: mediumTaj

src/IBM.WatsonDeveloperCloud/Constants.cs

@@ -26,21 +26,21 @@ public class Constants
         /// The version number for this SDK build. Added to the header in 
         /// each request as `User-Agent`.
         /// </summary>
-        public const string SDK_VERSION = "watson-apis-dotnet-sdk/2.8.0";
+        public static readonly string SDK_VERSION = "watson-apis-dotnet-sdk/2.8.0";
         /// <summary>
         /// A constant used to access custom request headers in the dynamic
         /// customData object.
         /// </summary>
-        public const string CUSTOM_REQUEST_HEADERS = "custom_request_headers";
+        public static readonly string CUSTOM_REQUEST_HEADERS = "custom_request_headers";
         /// <summary>
         /// A constant used to access response headers in the dynamic
         /// customData object.
         /// </summary>
-        public const string RESPONSE_HEADERS = "response_headers";
+        public static readonly string RESPONSE_HEADERS = "response_headers";
         /// <summary>
         /// A constnat used to access response json in the dynamic customData
         /// object.
         /// </summary>
-        public const string JSON = "json";
+        public static readonly string JSON = "json";
     }
 }