trim string value before compare in haskell-indent

in haskell-indent, 
```haskell
oneChar :: Char -> Doc
oneChar char = undefine
```
The above can be correctly indent by cycle-indent while the below can't
```haskell
oneChar      :: Char -> Doc
oneChar char = undefine
```

It is because we are using "string=" to compare "oneChar " and "oneChar      ", the indenter thinks they are not the same variables.