Make mergePairs work with one sample mergers

[derele]

mergePairs is still failing (producing unexpected output without an error or warning) when mergers were obtained for only one sample.

This was because at the end lenght one lists are simplified and then the nameing according to sample names was performed. This overwrote the names of the mergers dataframe and column one got the sample names.

Simple fix to name bofore simplifying the list.

[benjjneb]

Sorry for long delay in responding, but have been busy with my class. I should be able to get to this in the next week or two though.

[benjjneb]

So after a very long delay this is now fixed. To clarify, the bug behavior was produced by the following:

# dereplicate and dada a single paired-end sample
mm <- mergePairs(list(sam1=ddF), list(sam1=drpF), list(sam1=ddR), list(sam1=drpR))
names(mm)

[1] "sam1" NA NA NA NA NA NA NA NA

After this commit, this behaves as expected:

# dereplicate and dada a single paired-end sample
mm <- mergePairs(list(sam1=ddF), list(sam1=drpF), list(sam1=ddR), list(sam1=drpR))
names(mm)

[1] "sequence" "abundance" "forward" "reverse" "nmatch" "nmismatch" "nindel" "prefer" "accept"

Since it's been so long I just implemented the fix myself instead of taking your pull request to avoid the merge conflicts that have cropped up since then. But thanks again @derele for so clearly describing the problem and fix which made this very easy for me!