Fix issue ' cargo t --doc does not respect --color when a compilation…

… error occurs rust-lang#14403 '

src/cargo/ops/cargo_test.rs

@@ -214,6 +214,13 @@ fn run_doc_tests(
         gctx.shell().status("Doc-tests", unit.target.name())?;
         let mut p = compilation.rustdoc_process(unit, *script_meta)?;
 
+        let color_arg = match color {
+            ColorChoice::Always => "always",
+            ColorChoice::Never => "never",
+            ColorChoice::CargoAuto => "auto",
+        };
+        p.arg("--color").arg(color_arg);
+
         for (var, value) in env {
             p.env(var, value);
         }