format () {
	    this.version = `${this.major}.${this.minor}.${this.patch}`;
	    if (this.prerelease.length) {
	      this.version += `-${this.prerelease.join('.')}`;
	    }
	    return this.version
	  }
Set the client certificate key option for https request.

@param {Buffer | String} cert
@return {Request} for chaining
@api public