Handle 429 status code in response gracefully.

[m-aciek]

Closes #20.

[m-aciek]

@igusar-epam, @dyurchenko-epam could you take a look at it? That change would prevent us from internal library crash on internal check. And would allow us to not fork HW SDK, but use original one.

[m-aciek]

Now we just get:

hyperwallet.exceptions:HyperwalletAPIException: Invalid Content-Type specified in Response Header
Traceback (most recent call last):
...
File ".../hyperwallet/api.py", line 117, in getUser
File ".../hyperwallet/utils/apiclient.py", line 156, in doGet
File ".../hyperwallet/utils/apiclient.py", line 116, in _makeRequest
File ".../hyperwallet/utils/apiclient.py", line 208, in __checkResponseHeaderContentType

so the original cause is obscured.

[igusar-epam]

@m-aciek I think this change is ok. I can approve this pull request if you add me as a reviewer. Also you can add @aseveryn-hw to this discussion. He has a lot more experience in SDK programming.

[m-aciek]

@igusar-epam thank you! But it looks like I don't have permissions to assign a review :( [1].

[1] https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/requesting-a-pull-request-review

[igusar-epam]

@m-aciek I added myself as a reviewer and approved it. I also added other team members to it.

[m-aciek]

@vkorotchenko @wmews-hw @aseveryn-hw what do you think of this small change?

[m-aciek]

@wmews-hw Hi, I've corrected my pull request. Could you take a look?