function isDigit(subject) {
    var subjectString = coerceToString(subject);
    return REGEXP_DIGIT.test(subjectString);
  }
Checks whether `subject` contains only digit characters.

@function isDigit
@static
@since 1.0.0
@memberOf Query
@param {string} [subject=''] The string to verify.
@return {boolean} Returns `true` if `subject` contains only digit characters or `false` otherwise.
@example
v.isDigit('35');
// => true

v.isDigit('1.5');
// => false

v.isDigit('ten');
// => false