Add strict keyword argument to searches

[glostis]

According to the API documentation (https://developers.planet.com/docs/data/reference/#tag/Item-Search), it is possible to pass a strict argument to QuickSearches and SavedSearches.

This didn't seem to be implemented in the Python client, so I have added it.

I expect the user to pass in a call such as:

result = client.quick_search(request, strict=True)

with a Python boolean as the value for the keyword argument.

I do not know what low-level request-handling library you are using, but it seems that it does not automatically encode the params dict into a JSON string (for example, the library requests does that for you, if you call requests.post with a json keyword argument (documentation here).

In order to get around this problem, I json.dumps the value given, such that if you call _params(strict=True), it returns {"strict": "true"}.

[glostis]

I've rebased my branch on top of master.

Is there any chance someone could review this PR?

[sarasafavi]

@glostis much belated thanks for this! LGTM... will go out in the next release.