Don't quote the multipart boundary header. (dart-lang#63)

This is be more broadly compatible than the quoted version, although the
quoted version is what's described by the spec. curl and browsers send
unquoted boundaries, so we can safely assume that any server will
support them.

Closes dart-lang#61

Author: nex3

CHANGELOG.md

@@ -1,3 +1,13 @@
+## 0.11.3+12
+
+* Don't quote the boundary header for `MultipartRequest`. This is more
+  compatible with server quirks.
+
+## 0.11.3+11
+
+* Fix the SDK constraint to only include SDK versions that support importing
+  `dart:io` everywhere.
+
 ## 0.11.3+10
 
 * Stop using `dart:mirrors`.

lib/src/multipart_request.dart

@@ -83,7 +83,7 @@ class MultipartRequest extends BaseRequest {
   ByteStream finalize() {
     // TODO(nweiz): freeze fields and files
     var boundary = _boundaryString();
-    headers['content-type'] = 'multipart/form-data; boundary="$boundary"';
+    headers['content-type'] = 'multipart/form-data; boundary=$boundary';
     super.finalize();
 
     var controller = new StreamController<List<int>>(sync: true);
@@ -117,14 +117,19 @@ class MultipartRequest extends BaseRequest {
     return new ByteStream(controller.stream);
   }
 
-  /// All character codes that are valid in multipart boundaries. From
-  /// http://tools.ietf.org/html/rfc2046#section-5.1.1.
+  /// All character codes that are valid in multipart boundaries. This is the
+  /// intersection of the characters allowed in the `bcharsnospace` production
+  /// defined in [RFC 2046][] and those allowed in the `token` production
+  /// defined in [RFC 1521][].
+  ///
+  /// [RFC 2046]: http://tools.ietf.org/html/rfc2046#section-5.1.1.
+  /// [RFC 1521]: https://tools.ietf.org/html/rfc1521#section-4
   static const List<int> _BOUNDARY_CHARACTERS = const <int>[
-    39, 40, 41, 43, 95, 44, 45, 46, 47, 58, 61, 63, 48, 49, 50, 51, 52, 53, 54,
-    55, 56, 57, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80,
-    81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 97, 98, 99, 100, 101, 102, 103,
-    104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118,
-    119, 120, 121, 122
+    39, 43, 95, 44, 45, 46, 58, 61, 63, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57,
+    65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83,
+    84, 85, 86, 87, 88, 89, 90, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106,
+    107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121,
+    122
   ];
 
   /// Returns the header string for a field. The return value is guaranteed to