Make downcast to float explicit

Summary: Because explicit is better.

Reviewed By: sathyaphoenix

Differential Revision: D9834733

fbshipit-source-id: 6e6af1d5215021db522284b6c58c753522df20df

folly/Conv.h

@@ -694,7 +694,7 @@ toAppend(
       conv.ToShortest(value, &builder);
       break;
     case DoubleToStringConverter::SHORTEST_SINGLE:
-      conv.ToShortestSingle(value, &builder);
+      conv.ToShortestSingle(static_cast<float>(value), &builder);
       break;
     case DoubleToStringConverter::FIXED:
       conv.ToFixed(value, int(numDigits), &builder);