Implement Response::json for wasm32 target (#802)

Cargo.toml

@@ -62,6 +62,9 @@ __internal_proxy_sys_no_cache = []
 http = "0.2"
 url = "2.1"
 bytes = "0.5"
+serde = "1.0"
+## json
+serde_json = { version = "1.0", optional = true }
 
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
 base64 = "0.11"
@@ -82,7 +85,6 @@ pin-project-lite = "0.1.1"
 
 # TODO: candidates for optional features
 
-serde = "1.0"
 serde_urlencoded = "0.6.1"
 
 # Optional deps...
@@ -105,8 +107,6 @@ cookie_store = { version = "0.10", optional = true }
 ## gzip
 async-compression = { version = "0.2.0", default-features = false, features = ["gzip", "stream"], optional = true }
 
-## json
-serde_json = { version = "1.0", optional = true }
 
 ## socks
 tokio-socks = { version = "0.2", optional = true }