GH #248: Fix GreenFileIO.write()

On Python 3, GreenFileIO.write() now always write all bytes using a
loop, to have the same behaviour than GreenPipe.write() on Python 2.
Before, the write() could be partial, write less bytes than expected.

On Python 2, GreenPipe.write() doesn't allow partial writes, it
always write all bytes, even if the user requested an unbuffered pipe
(ex: by calling os.fdopen(fd, 'wb', 0)).

Modifying Python 2 to be pedantic and allow partial writes for
unbuffered pipes will likely break a lot of applications. It's
simpler to modify Python 3 GreenFileIO to behave the same than Python
2 GreenPipe.

eventlet/greenio/py3.py

@@ -132,14 +132,19 @@ class GreenFileIO(_OriginalIOBase):
         self._closed = True
 
     def write(self, data):
-        while True:
+        view = memoryview(data)
+        datalen = len(data)
+        offset = 0
+        while offset < datalen:
             try:
-                return _original_os.write(self._fileno, data)
+                written = _original_os.write(self._fileno, view[offset:])
             except OSError as e:
                 if get_errno(e) not in SOCKET_BLOCKING:
                     raise IOError(*e.args)
-                else:
-                    trampoline(self, write=True)
+                trampoline(self, write=True)
+            else:
+                offset += written
+        return offset
 
     def close(self):
         if not self._closed: