fix ios formdata boundary include slash

Summary:
some server not work when upload a file with FromData in ios.
the reason is that there is a slash in boundary, like:
```
multipart/form-data; boundary=b/QeEbFgqK9PCZo4T/eXv7f.T74SHd5MxCZ846AsTz-yNV0xrRR_Zks4fkNMCzJck9ZE8o

// koa request.js (line 548)
  is(types) {
    if (!types) return typeis(this.req);
    if (!Array.isArray(types)) types = [].slice.call(arguments);
    return typeis(this.req, types);
  }

// type-is index.js (line 237)
function normalizeType (value) {
  // parse the type
  var type = typer.parse(value)

  // remove the parameters
  type.parameters = undefined

  // reformat it
  return typer.format(type)
}

// media-typer
var paramRegExp = /; *([!#$%&'\*\+\-\.0-9A-Z\^_`a-z\|~]+) *= *("(?:[ !\u0023-\u005b\u005d-\u007e\u0080-\u00ff]|\\[\u0020-\u007e])*"|[!#$%&'\*\+\-\.0-9A-Z\^_`a-z\|~]+) */g;
```

thanks for dougwilson 's [answer](https://github.com/jshttp/media-typer/issues/5).
> The / is an illegal character for Content-Type, which is what this module parses
Closes https://github.com/facebook/react-native/pull/11342

Differential Revision: D4326750

Pulled By: javache

fbshipit-source-id: b1c78a335c95a5c223537545d87beaffe15d673d

Libraries/Network/RCTNetworking.mm

@@ -49,7 +49,7 @@ typedef RCTURLRequestCancellationBlock (^RCTHTTPQueryResult)(NSError *error, NSD
 static NSString *RCTGenerateFormBoundary()
 {
   const size_t boundaryLength = 70;
-  const char *boundaryChars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_./";
+  const char *boundaryChars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_.";
 
   char *bytes = (char*)malloc(boundaryLength);
   size_t charCount = strlen(boundaryChars);