Return strings instead of null

Author: andyneff

rest_framework/utils/encoders.py

@@ -59,11 +59,11 @@ def floatstr(o, allow_nan=self.allow_nan,
             # internals.
 
             if o != o:
-                text = 'null'
+                text = '"NaN"'
             elif o == _inf:
-                text = 'null'
+                text = '"Infinity"'
             elif o == _neginf:
-                text = 'null'
+                text = '"-Infinity"'
             else:
                 return _repr(o)