Spec fails with libmongoc 1.4.2 (Debian stretch)

[Val]

Environment: a Debian stretch up to date, with libmongoc-dev package

> crystal --version
Crystal 0.22.0 [3c71228] (2017-04-20) LLVM 3.5.0
> llvm-config-3.9 --version
3.9.1
> cat /etc/{issue,debian_version}
Debian GNU/Linux 9 \n \l

9.0
> dpkg -l libmongoc-dev
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                                            Version                      Architecture                 Description
+++-===============================================-============================-============================-===================================================================================================
ii  libmongoc-dev                                   1.4.2-1+b1                   amd64                        MongoDB C client library - dev files

From mongo.cr master branch.

> git clone https://github.com/datanoise/mongo.cr
Cloning into 'mongo.cr'...
remote: Counting objects: 355, done.
remote: Total 355 (delta 0), reused 0 (delta 0), pack-reused 354
Receiving objects: 100% (355/355), 78.30 KiB | 0 bytes/s, done.
Resolving deltas: 100% (203/203), done.
Checking connectivity... done.
> crystal spec
FF......................................................................

Failures:

  1) Mongo::Uri should be able to create new uri
     Failure/Error: host.port.should eq(27017)

       Expected: 27017
            got: 0

     # spec/uri_spec.cr:10

  2) Mongo::Uri should be able to create new uri with host and port
     Failure/Error: host.port.should eq(27017)

       Expected: 27017
            got: 0

     # spec/uri_spec.cr:18

Finished in 590.46 milliseconds
72 examples, 2 failures, 0 errors, 0 pending

Failed examples:

crystal spec spec/uri_spec.cr:5 # Mongo::Uri should be able to create new uri
crystal spec spec/uri_spec.cr:13 # Mongo::Uri should be able to create new uri with host and port