BUG: Fixed grow_buffer to grow when capacity is reached
Addresses issue in #12494 by allowing `grow_buffer` to grow the size of the parser buffer when buffer capacity is achieved.  Previously, you had to exceed capacity for this to occur, but that was inconsistent with the `end_field` check later on when handling the EOF terminator, where reached capacity was considered a buffer overflow.

Tests are passing.  Should be good to merge, given how simple the fix was.
 thank you sir!
 Could you also close the associated issue?  It doesn't seem to have been closed with the commit you made.
 hmm, `addresses #issue number` should have worked (I usually put ``#closes in the top of the pr to have this done automagically)
