Fixed compiler errors when compiling http-server.

I needed to add some Data.Text.pack/Data.Text.unpack calls in the HtmlForm module. Therefore I also had to add the `text` module to the build dependencies in the Cabal file.

Now the compile process runs fine, although I did not actually test the changes; however, as they only fix some type errors it should still work fine.

The errors displayed by GHC 7.6.3 were:

```
Network/HTTP/Server/HtmlForm.hs:64:55:
    Couldn't match type `text-1.0.0.0:Data.Text.Internal.Text'
                  with `[Char]'
    Expected type: String
      Actual type: text-1.0.0.0:Data.Text.Internal.Text
    In the first argument of `decodeString', namely `k'
    In the expression: decodeString k
    In the expression: (decodeString k, v)

Network/HTTP/Server/HtmlForm.hs:75:22:
    Couldn't match type `(String, String)' with `MIMEParam'
    Expected type: [MIMEParam]
      Actual type: [(String, String)]
    In the first argument of `parseMIMEBody', namely `hdrs'
    In the expression: parseMIMEBody hdrs body
    In the expression:
      let
        hdrs = map (\ (Header a b) -> ...) (rqHeaders req)
        body = rqBody req
      in parseMIMEBody hdrs body

Network/HTTP/Server/HtmlForm.hs:75:27:
    Couldn't match type `[Char]'
                  with `text-1.0.0.0:Data.Text.Internal.Text'
    Expected type: text-1.0.0.0:Data.Text.Internal.Text
      Actual type: String
    In the second argument of `parseMIMEBody', namely `body'
    In the expression: parseMIMEBody hdrs body
    In the expression:
      let
        hdrs = map (\ (Header a b) -> ...) (rqHeaders req)
        body = rqBody req
      in parseMIMEBody hdrs body
```