Adds an attribute to a subrange of the string.

Parameters:

attribute     the attribute key

value     The value of the attribute. May be null.

beginIndex     Index of the first character of the range.

endIndex     Index of the character following the last character of the range.

Exceptions:

NullPointerException if attribute is null.

IllegalArgumentException if beginIndex is less then 0, endIndex is greater than the length of the string, or beginIndex and endIndex together don't define a non-empty subrange of the string.

`attribute`		the attribute key
`value`		The value of the attribute. May be null.
`beginIndex`		Index of the first character of the range.
`endIndex`		Index of the character following the last character of the range.

`NullPointerException`		if `attribute` is null.
`IllegalArgumentException`		if beginIndex is less then 0, endIndex is greater than the length of the string, or beginIndex and endIndex together don't define a non-empty subrange of the string.