Corrected multipart boundary size so it match firefox boundaries.

On 64bits systems, mechanize as been generating very long boundary which
are blocked by certains firewall.
This commit restore the behaviors of 32bits system on 64bits system

Author: ixeft

mechanize/_form_controls.py

@@ -72,7 +72,7 @@ def isstringlike(x):
 def choose_boundary():
     """Return a string usable as a multipart boundary."""
     # follow IE and firefox
-    nonce = "".join(str(random.randint(0, sys.maxsize - 1)) for i in (0, 1, 2))
+    nonce = "".join(str(random.randint(0, 2**31 - 1)) for i in (0, 1, 2))
     return "-" * 27 + nonce