Creating (but not sending) a Message(::String) crashes on Windows

[rdeits]

I'm afraid I have absolutely no idea why, but adding this line to runtests.jl causes the test to crash:

msg = Message("hello")

with the error message:

Assertion failed!
Program: C:\projects\julia\bin\julia.exe
File: /home/Administrator/buildbot/worker/package_win32/build/deps/srccache/libuv-d8ab1c6a33e77bf155facb54215dd8798e13825d/src/win/async.c, Line 77
Expression: !(handle->flags & UV__HANDLE_CLOSING)
This application has requested the Runtime to terminate it in an unusual way.
Please contact the application's support team for more information.
signal (22): SIGABRT
while loading no file, in expression starting on line 0
raise at C:\windows\SYSTEM32\msvcrt.dll (unknown line)
abort at C:\windows\SYSTEM32\msvcrt.dll (unknown line)
_wassert at /usr/src/debug/mingw64-i686-runtime-5.0.2-1/misc\wassert.c:54
free at C:\windows\SYSTEM32\msvcrt.dll (unknown line)
free at C:\windows\SYSTEM32\msvcrt.dll (unknown line)
malloc at C:\windows\SYSTEM32\msvcrt.dll (unknown line)
Znwj at C:\projects\julia\bin\libstdc++-6.dll (unknown line)
uv_close at /home/Administrator/buildbot/worker/package_win32/build/deps/srccache/libuv-d8ab1c6a33e77bf155facb54215dd8798e13825d/src/win\handle.c:124
RtlAllocateHeap at C:\windows\SYSTEM32\ntdll.dll (unknown line)
malloc at C:\windows\SYSTEM32\msvcrt.dll (unknown line)
_assert at /usr/src/debug/mingw64-i686-runtime-5.0.2-1/misc\wassert.c:30
_M_construct<char*> at /home/Administrator/buildbot/worker/package_win32/build/src/usr/lib/gcc/i686-w64-mingw32/5.4.0/include/c++/bits\basic_string.tcc:223
free at C:\windows\SYSTEM32\msvcrt.dll (unknown line)
uv_async_send at /home/Administrator/buildbot/worker/package_win32/build/deps/srccache/libuv-d8ab1c6a33e77bf155facb54215dd8798e13825d/src/win\async.c:77
gc_free_fn at C:\Users\appveyor\.julia\v0.6\ZMQ\src\ZMQ.jl:315
unknown function (ip: 08C72A5B)
unknown function (ip: 6998A7BD)
unknown function (ip: 08C7F312)

This is the root cause of the appveyor failures seen in #165

Example: https://ci.appveyor.com/project/tkelman/zmq-jl-dvf4g/build/job/jf9vqk4umt8v6993