Ignore ndigits passed to BigDecimal(string, ndigits)

[tompng]

Fixes #355
Don't allocate huge memory when huge ndigits is passed.

Document:

/*
 *      # String does not require ndigits; ignored if given.
 *      BigDecimal('2')     # => 0.2e1
 *      BigDecimal('2.0')   # => 0.2e1
 *      BigDecimal('0.2e1') # => 0.2e1
 *      BigDecimal(' 2.0 ') # => 0.2e1
 */