response.buffer() and blob.type have no/limited access in ReactNative

response.buffer() is not supported at all, and blob.type is readonly in
ReactNative. Change the limited node/browser detection to (hopefully)
feature-proof feature detection instead.

Errors fixed in RN:

    Creating blobs from 'ArrayBuffer' and 'ArrayBufferView' are not
    supported

    Cannot set property type of #<Blob> which has only a getter at

Fixes pouchdb#7688 and pouchdb#7727

packages/node_modules/pouchdb-adapter-http/src/index.js

@@ -506,17 +506,17 @@ function HttpPouch(opts, callback) {
         var path = encodeDocId(doc._id) + '/' + encodeAttachmentId(filename) +
             '?rev=' + doc._rev;
         return ourFetch(genDBUrl(host, path)).then(function (response) {
-          if (typeof process !== 'undefined' && !process.browser) {
+          if ('buffer' in response) {
             return response.buffer();
           } else {
             /* istanbul ignore next */
             return response.blob();
           }
         }).then(function (blob) {
           if (opts.binary) {
-            // TODO: Can we remove this?
-            if (typeof process !== 'undefined' && !process.browser) {
-              blob.type = att.content_type;
+            var typeFieldDescriptor = Object.getOwnPropertyDescriptor(blob, 'type');
+            if (typeFieldDescriptor && typeFieldDescriptor.set) {
+              blob.type = contentType;
             }
             return blob;
           }
@@ -875,7 +875,7 @@ function HttpPouch(opts, callback) {
     if (opts.descending) {
       params.descending = true;
     }
-    
+
     /* istanbul ignore if */
     if (opts.update_seq) {
       params.update_seq = true;