LibCore: Make IODevice::read_line() return a String

Almost everyone using this API actually wanted String instead of a
ByteBuffer anyway, and there were a bunch of slightly different ways
clients would convert to String.

Let's just cut out all the confusion and make it return String. :^)

Libraries/LibTLS/Socket.cpp

@@ -53,7 +53,7 @@ ByteBuffer TLSv12::read(size_t max_size)
     return {};
 }
 
-ByteBuffer TLSv12::read_line(size_t max_size)
+String TLSv12::read_line(size_t max_size)
 {
     if (!can_read_line())
         return {};
@@ -70,7 +70,7 @@ ByteBuffer TLSv12::read_line(size_t max_size)
     auto buffer = ByteBuffer::copy(start, offset);
     m_context.application_buffer = m_context.application_buffer.slice(offset + 1, m_context.application_buffer.size() - offset - 1);
 
-    return buffer;
+    return String::copy(buffer);
 }
 
 bool TLSv12::write(const ByteBuffer& buffer)