Consistent grid sizes in streamplot.

This PR addresses #2653. The grid size used by `streamplot()` for `density=1` was 30x30 (not 25x25 as stated in the docstring) but for `density=(1, 1)` it was 25x25. This PR changes the base grid size for both density specification types to 30x30, since this is the grid size currently used for the default value of the `density` argument.

I modified two tests that use a vector density specification so that the resulting test plots are the same. I saw no need for new test images taking up space in the repository.