Coerce inputs to startsWith and endsWith

[mikmart]

Inputs to functions for performing character operations, like substr() and grep(), are typically coerced to character values. However, startsWith() and endsWith() throw errors for non-character inputs, instead. This is particularly notable with factors, for example startsWith(factor("foo"), "f") is currently an error. This patch adds coercion to these two functions, consistent with other functions.