Use boost::filesystem::path::string() instead of path::native()

This fixes a mingw32 conversion error during cross-compilation.

src/gtest/test_paymentdisclosure.cpp

@@ -99,7 +99,7 @@ TEST(paymentdisclosure, mainnet) {
     boost::filesystem::create_directories(pathTemp);
     mapArgs["-datadir"] = pathTemp.string();
 
-    std::cout << "Test payment disclosure database created in folder: " << pathTemp.native() << std::endl;
+    std::cout << "Test payment disclosure database created in folder: " << pathTemp.string() << std::endl;
 
     PaymentDisclosureDBTest mydb(pathTemp);
 

src/test/rpc_wallet_tests.cpp

@@ -421,7 +421,7 @@ BOOST_AUTO_TEST_CASE(rpc_wallet_z_exportwallet)
     BOOST_CHECK_THROW(CallRPC(string("z_exportwallet ") + tmpfilename.string()), runtime_error);
 
     // set exportdir
-    mapArgs["-exportdir"] = tmppath.native();
+    mapArgs["-exportdir"] = tmppath.string();
 
     // run some tests
     BOOST_CHECK_THROW(CallRPC("z_exportwallet"), runtime_error);
@@ -501,7 +501,7 @@ BOOST_AUTO_TEST_CASE(rpc_wallet_z_importwallet)
     // write test data to file
     boost::filesystem::path temp = boost::filesystem::temp_directory_path() /
             boost::filesystem::unique_path();
-    const std::string path = temp.native();
+    const std::string path = temp.string();
     std::ofstream file(path);
     file << testWalletDump;
     file << std::flush;