Clear interactions when setting mode

The previous commit cleared interactions every time a new interaction
was recorded. That meant that only a single interaction could ever be
saved.

Here we clear interactions once when the mode is set. This'll happen
after interactions have been loaded, since `VCRMiddleware` is
instantiated using `TryFrom<PathBuf>` which loads the old interactions.

src/lib.rs

@@ -88,6 +88,10 @@ pub type VCRError = &'static str;
 impl VCRMiddleware {
     /// Adjust mode in the middleware and return it
     pub fn with_mode(mut self, mode: VCRMode) -> Self {
+        if mode == VCRMode::Record {
+            let mut cassette = self.storage.lock().unwrap();
+            cassette.http_interactions.clear();
+        }
         self.mode = mode;
         self
     }
@@ -377,7 +381,6 @@ impl VCRMiddleware {
 
     fn record(&self, request: vcr_cassette::Request, response: vcr_cassette::Response) {
         let mut cassette = self.storage.lock().unwrap();
-        cassette.http_interactions.clear();
         cassette
             .http_interactions
             .push(vcr_cassette::HttpInteraction {
@@ -482,7 +485,7 @@ impl Drop for VCRMiddleware {
     }
 }
 
-/// Load VCR cassette for filesystem
+/// Load VCR cassette from filesystem
 //
 /// For simplicity, support JSON format only for now
 impl TryFrom<PathBuf> for VCRMiddleware {