PDBWriter uses first character of segid as chainID instead of the last as written in the documentation

[capslockwizard]

Expected behavior
When writing out a PDB file with MDAnalysis, the chains IDs in the PDB file are taken from the last character of the segid as documented here.

Actual behavior
Instead, the first character from segid is used as the chain ID in PDB files. From the source:

vals['chainID'] = segids[i][:1]

Code to reproduce the behavior

import MDAnalysis as mda
from MDAnalysis.tests.datafiles import PSF, DCD,  GRO, PDB, TPR, XTC, TRR,  PRMncdf, NCDF

u = mda.Universe(PSF, DCD)
u.write('test.pdb')

Current version of MDAnalysis

• Which version are you using? 0.19.2
• Which version of Python (python -V)? 2.7.15
• Which operating system? Ubuntu 16.04