Prevent inspect from being called recursively

This is related to the problems with 2.0.0 and the Spreadsheet gem.

It appears this simple fix will prevent absurdly long load times on calls like:

```spreadsheet = Roo::Excel.new("foo.xls")```
as opposed to:

```Roo::Excel.new("foo.xls").first_line```
which always ran quite quickly.

The difference is the first version, when called in a program like IRB implicitly calls inspect, which (I think?) recursively calls inspect on all the nested instance variables, again, I think. Even files of a relatively small size (165kb xls) were taking hours to load before I made this change, now it takes a brief second.