PDB writer does not work when an xtc is loaded

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What steps will reproduce the problem?
1. U=Universe(PDB,XTC)
2. U.trajectory[x]
3. U.atoms.write("frame_X",format="PDB")

What is the expected output? What do you see instead?
A PDB file containing the selected frame from the xtc is expected but instead 
the trajectory is written to the pdb.

What version of the product are you using? On what operating system?
MDAnalysis-0.7.5.1-py2.7-macosx-10.6-i386

Please provide any additional information below.
The problem occurs either when using a selection .write() method or when 
creating a PDBWriter instance. 
There also appears to be a discrepancy between the doc and the code regarding 
the number of arguments taken by the writer when trying to specify a specific 
frame to write.
There is no problem however with the GROWriter/GRO format.

Original issue reported on code.google.com by Jean.He...@gmail.com on 8 May 2012 at 10:30

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Jean, many thanks for filing the report!

Original comment by orbeckst on 8 May 2012 at 8:08

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Original comment by orbeckst on 24 May 2012 at 5:37

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This issue was closed by revision 2d2427efbd4a.

Original comment by orbeckst on 25 May 2012 at 8:09

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Issue 114 has been merged into this issue.

Original comment by orbeckst on 8 Jun 2012 at 6:24

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