MapMarkers does not generate the color staticmap parameter correctly

[MaximillianC]

When setting the Color property on the a MapMarkers object, the resulting URL calculated does not include the necessary "color:" prefix, resulting in the URL being invalid.

gmaps-api-net/src/Google.Maps/StaticMaps/StaticMapRequest.cs

Line 368 in c0f0575

 sb.AppendFormat(current.Color.To24BitColorString());