Fix premature truncation of the recurrence sum in zeta(s,z). #489
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As descriped in issue #488, skipping the recurrence sum or truncating early causes the function to break inside a region determined by shape of the cutoff. This rewrite also fixes another bug I encountered: sometimes the zero denominator term wasn't actually skipped, which caused sporadic failures.
This plot shows an example of how the error is improved relative to$(2^s-1)\zeta(s)$ in the affected area:

The accuracy still isn't perfect but the major breakage is fixed. Hopefully it can be improved in the future.