login: Send arguments separately to npm

All arguments were passed to `arg` inside one string when building a
`Command`. However, using the `arg` function, "only one argument can be
passed per use." This caused all arguments accidentally to be appended
to the registry URL. For example: After a successful login with a
provided `--auth_type`, the success message incorrectly displayed:
"Logged in as asf on https://registry.npmjs.org/%20--auth_type=Basic."
The space (%20 in hex) was caused by adding a fixed space before each
additional argument.

This commit pushes all arguments onto a `Vec<String>`. Then, the `args`
function adds the arguments separately to the command. This removes the
need to prepend spaces to each argument. Alternatively, `arg` could have
been used throughout to build the command argument-by-argument. However,
using `args` partitions the code more neatly into two distinct sections.

src/npm.rs

@@ -46,26 +46,24 @@ pub fn npm_login(
     always_auth: bool,
     auth_type: &Option<String>,
 ) -> Result<(), failure::Error> {
-    let mut args = String::new();
-
-    args.push_str(&format!("--registry={}", registry));
+    let mut args = vec![format!("login"), format!("--registry={}", registry)];
 
     if let Some(scope) = scope {
-        args.push_str(&format!(" --scope={}", scope));
+        args.push(format!("--scope={}", scope));
     }
 
     if always_auth == true {
-        args.push_str(" --always_auth");
+        args.push(format!("--always_auth"));
     }
 
     if let Some(auth_type) = auth_type {
-        args.push_str(&format!(" --auth_type={}", auth_type));
+        args.push(format!("--auth_type={}", auth_type));
     }
 
     // Interactively ask user for npm login info.
     //  (child::run does not support interactive input)
     let mut cmd = Command::new("npm");
-    cmd.arg("login").arg(args);
+    cmd.args(args);
 
     info!(log, "Running {:?}", cmd);
     match cmd.status()?.success() {