fix(did-resolver): ensure content-type is text/plain and is a did

closes #33

did-resolver/src/lib.rs

···
 use error::Error;
 use hickory_resolver::proto::serialize::binary::BinEncodable;
 use hickory_resolver::TokioAsyncResolver;
+use reqwest::header::CONTENT_TYPE;
 use reqwest::{Client, StatusCode};
 
 pub mod error;
···
             .await?
             .error_for_status()?;
 
+        if let Some(ct) = res.headers().get(CONTENT_TYPE) {
+            if ct != "text/plain" {
+                return Ok(None);
+            }
+        }
+
         let did = res.text().await?;
+        if !did.starts_with("did:") {
+            return Ok(None);
+        }
 
         Ok(Some(did))
     }