Enforce `‎SerpResults.yield_pages` to start from the first page

[ilyazub]

Currently, ‎SerpResults.yield_pages skips the first page. Because of that, it's required to duplicate the results processing in the clients' code.

serpapi-python/serpapi/models.py

Lines 74 to 78 in 44e41d3

	current_page = self
	while current_page.next_page_url and current_page_count < max_pages:
	current_page = current_page.next_page()
	current_page_count += 1
	yield current_page

Let's yield current_page before entering the pagination loop.

@kennethreitz @jvmvik

[hilmanski]

@ilyazub I tried to update the method in this PR #13 , as well as the test to be more precise.