Emitter: produce string with trailing '.0' for floating point value with only integral part

[romainreignier]

For example the number 1.0 will be converted to "1.0" instead of "1".
This allows to keep the correct type information when parsing back the YAML.

This is an attempt to Fix #226 #412 #1016

[romainreignier]

I don't know if it worth adding it, but this test fail on master and pass with this patch:

void testRoundTripDouble(double value) {
    Emitter out;
    out << value;
    Node node = Load(out.c_str());
	int64_t intValue;
	EXPECT_FALSE(YAML::convert<int64_t>::decode(node, intValue));
	double doubleValue;
	EXPECT_TRUE(YAML::convert<double>::decode(node, doubleValue));
    EXPECT_EQ(doubleValue, value);
}

TEST(RoundTripTest, Double) {
    testRoundTripDouble(0.0);
    testRoundTripDouble(3.14159);
}