Add conversion/comparison operators to Buffer

tests/buffer_test.cpp

@@ -0,0 +1,62 @@
+#include <string>
+#include <vector>
+#include <sstream>
+#include <gtest/gtest.h>
+#include "cppkafka/buffer.h"
+
+using std::string;
+using std::vector;
+using std::ostringstream;
+
+using namespace cppkafka;
+
+class BufferTest : public testing::Test {
+public:
+    
+};
+
+TEST_F(BufferTest, StringConversion) {
+    string data = "Hello world!";
+    Buffer buffer(data);
+    string buffer_as_string = buffer;
+    EXPECT_EQ(data, buffer_as_string);
+}
+
+TEST_F(BufferTest, StringConversionOnEmptyBuffer) {
+    Buffer buffer;
+    EXPECT_EQ("", static_cast<string>(buffer));
+}
+
+TEST_F(BufferTest, VectorConversion) {
+    string data = "Hello world!";
+    Buffer buffer(data);
+    vector<char> buffer_as_vector = buffer;
+    EXPECT_EQ(data, string(buffer_as_vector.begin(), buffer_as_vector.end()));
+}
+
+TEST_F(BufferTest, Equality) {
+    string data = "Hello world!";
+    Buffer buffer1(data);
+    Buffer buffer2(data);
+
+    EXPECT_EQ(buffer1, buffer2);
+}
+
+TEST_F(BufferTest, InEquality) {
+    string data1 = "Hello world!";
+    string data2 = "Hello worldz";
+    Buffer buffer1(data1);
+    Buffer buffer2(data2);
+
+    EXPECT_NE(buffer1, buffer2);
+}
+
+TEST_F(BufferTest, OutputOperator) {
+    string data = "Hello \x7fwor\x03ld!";
+    string pretty_string = "Hello \\x7fwor\\x03ld!";
+    Buffer buffer(data);
+
+    ostringstream output;
+    output << buffer;
+    EXPECT_EQ(pretty_string, output.str());
+}