toAppend support DoubleToStringConverter::SHORTEST_SINGLE (#928)

Summary:
toAppend support DoubleToStringConverter::SHORTEST_SINGLE for float type.
Pull Request resolved: https://github.com/facebook/folly/pull/928

Reviewed By: yfeldblum

Differential Revision: D9790564

Pulled By: Orvid

fbshipit-source-id: b38d6542518aa0ed7f2b0e222e9870bb5f3a51e5

folly/Conv.h

@@ -693,6 +693,9 @@ toAppend(
     case DoubleToStringConverter::SHORTEST:
       conv.ToShortest(value, &builder);
       break;
+    case DoubleToStringConverter::SHORTEST_SINGLE:
+      conv.ToShortestSingle(value, &builder);
+      break;
     case DoubleToStringConverter::FIXED:
       conv.ToFixed(value, int(numDigits), &builder);
       break;