Auto merge of #1657 - jtgeibel:update/serde, r=jtgeibel

Update serde crates and their dependencies

`cargo update -p serde -p serde_json -p serde_urlencoded -p serde_derive --aggressive`

All serde crates and their dependencies are updated:

```
    Updating dtoa v0.4.2 -> v0.4.3
    Updating idna v0.1.4 -> v0.1.5
    Removing itoa v0.3.4
    Updating matches v0.1.6 -> v0.1.8
    Removing num-traits v0.1.40
      Adding num-traits v0.1.43
      Adding num-traits v0.2.6
    Updating percent-encoding v1.0.0 -> v1.0.1
    Updating proc-macro2 v0.4.6 -> v0.4.27
    Updating quote v0.6.3 -> v0.6.11
      Adding ryu v0.2.7
    Updating serde v1.0.80 -> v1.0.89
    Updating serde_derive v1.0.80 -> v1.0.89
    Updating serde_json v1.0.5 -> v1.0.39
    Updating serde_urlencoded v0.5.3 -> v0.5.4
    Updating syn v0.15.13 -> v0.15.29
    Updating unicode-normalization v0.1.5 -> v0.1.8
    Updating url v1.6.0 -> v1.7.2
```

Author: bors