v1.5.22 - Fix minor bug in `import_mothur`

https://github.com/joey711/phyloseq/issues/242
phyloseq 1.5.22 - Fix minor bug in `import_mothur`
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- Bug occurs only when sample names are pure integers. One line fix.
- This addresses [Issue 242](https://github.com/joey711/phyloseq/issues/242)