Add percent_encode_noop configuration to Form (#467)

Some implementations of multipart/form parsing don't support alternate
encodings, but do allow unicode in 'standard' parameter values. This
change allows users to specify that they do not want to encode
parameters.

(See also: #419, #420.)

src/multipart.rs

@@ -130,6 +130,11 @@ impl Form {
         self.with_inner(|inner| inner.percent_encode_attr_chars())
     }
 
+    /// Configure this `Form` to skip percent-encoding
+    pub fn percent_encode_noop(self) -> Form {
+        self.with_inner(|inner| inner.percent_encode_noop())
+    }
+
     pub(crate) fn reader(self) -> Reader {
         Reader::new(self)
     }